Having a debit card with a Visa/MC logo will make life much easier, since they are near-universally accepted. Look at your card and see what networks it's a part of. Cirrus is one...I forget the others. If it has one of those network logos, then you can use it at any ATM with that logo. I think Cirrus is nearly the same as MasterCard and fairly universal. Not sure about others. What's the credit union?
If you're going to rely on a debit card for cash, it would make sense to open a checking account with a different bank that issues a Visa/MC branded debit card for this trip. I use
Ally Bank, an online-only bank that issues a MasterCard debit card. They don't charge fees for most things and you can do everything online. They do need you to tell them if you are going overseas, however. I didn't do it when I went to Peru. My card worked once and then the next ATM I used swallowed it and I had to wait until I got home for a replacement.
edit: it's worth noting that Ally charges 1% fee for foreign ATM transactions